The present invention relates to a transformer isolated driver, and more particularly to a transformer isolated gate driver with an active switch (For example: a small power MOSFET) so as to achieve reliable transient performance.
The transformer isolated drivers are used for the driving of the high side power switch (For example: a MOSFET). Please refer to FIG. 1, which is the circuit of a transformer isolated driver of the prior art. T1 is an isolation transformer, QM is a switch (a MOSFET), the capacitor C1 is an input capacitor of the transformer T1, and the reference directions of the voltages on the capacitors are shown in FIG. 1, and C3 is an equivalent input capacitor of the switch QM.
FIG. 2 is the operating waveforms of the circuit shown in FIG. 1. S1 is the waveform of the output signal of the pulse width modulation (PWM) driver, S2 is the waveform of the primary winding of the transformer, and S3 is the output waveform of the transformer. Assuming that in the steady state, the period of the output signal of the PWM driver S1 is T, the duty-cycle is D, the amplitude value is V1, and the turns ratio of the transformer T1 is 1, then the steady state voltage on the input DC isolation capacitor C1 is DV1. When S1 is high, S3 is also high, and the amplitude of S3 is (V1xe2x88x92VC1), that is (1xe2x88x92D)V1. When S1 is low, S2 is negative, and the amplitude of S2 is (xe2x88x92VC1), that is xe2x88x92DV1. Thus, the switch QM has a reversed gate driving voltage so as to have a strong characteristic of anti-interference. However, a shortcoming of the circuit is that when the duty cycle D is larger, the amplitude of S3, which is (1xe2x88x92D)V1, becomes smaller, which might cause the insufficient driving of the switch QM. Therefore, the circuit of this kind of drivers is not suitable for the situations having larger variations of duty cycle.
Please refer to FIG. 3, which is another transformer isolated driver of the prior art. The polarities of the transformer T1 are shown in FIG. 3, capacitor C1 is an input capacitor at the primary side of the transformer, capacitor C2 is an output capacitor at the secondary side of the transformer, and C3 is the equivalent input capacitor of a power switch QM (a MOSFET). Please refer to FIG. 4 for operating waveforms of the circuit shown in FIG. 3, S1 is the waveform of the output signals of the PWM driver, S2 is the waveform on the primary of the transformer, and S3 is the output waveform of the transformer. Assuming that in the steady state, the period of the output signal of the PWM driver S1 is T, the duty-cycle is D, the amplitude value of S1 is V1, and the turn ratio of the transformer is 1, then the voltage of steady state across the input capacitor C1 at steady state is VC1=DV1, the voltage across the output capacitor C2 is VC2=DV1, and the reference directions of the voltages across these two capacitors are shown in FIG. 3. When S1 is high, S2 and S3 are both high. The amplitude of S2 is V2=(V1xe2x88x92VC1), that is. (1xe2x88x92D)V1. The amplitude of S3 is V3=(V2+VC2), that is V1. Thus the amplitude of S3 has no relationship with the duty-cycle D, and keeps unchanged as V1 at any duty-cycle operating condition. When S1 becomes zero, the input/output voltages of the transformer becomes xe2x88x92DV1, thus the diode DR is conducted, and the voltage across the equivalent input capacitor C3 of the switch QM is kept zero.
However, the still existed drawback of the above-mentioned transformer isolated drivers is: that when the driving signal S1 is disappeared due to certain reasons, S3 will become zero immediately as mentioned above. But latter on, the input of the transformer T1 is crossed by the bias of voltage (xe2x88x92DV1) from the capacitor C1 , and will be saturated gradually, thus the amplitude of the input/output voltages of the transformer T1 will become smaller starting from DV1. Since the voltage across the capacitor C2, is still DV1, therefore, the voltage across the equivalent input capacitor C3 of the switch QM will be increased starting from zero. Until the transformer T1 is saturated, then the primary and secondary side voltages of the transformer T1 will become zero, and the voltage across C3 will finally become DV1, which will cause the switch QM to suffer a mis-triggering in a longer time resulting in circuit damage. Please refer to FIG. 5 for the recorded operating waveform of S3 under circuit protection operating. Therefore, this kind of driving circuits does not have a better transient operating. When a power converter implemented with this driver is started up or is shut down, the power switch will suffer a mis-triggering, thus causing the break down of the whole power converter.
It is therefore an object of the present invention to provide a transformer isolated gate driver with an active switch (a small power MOSFET) for driving of a power switch with reliable transient performance, and avoiding the power switch from suffering mis-triggering so as to prevent the break down of the whole power converter.
According to the aspect of the present invention, the transformer isolated gate driver electrically connected to a power switch for realizing the driving of the power switch includes: a PWM driver for producing a PWM signal; a transformer electrically connected to the PWM driver; an input capacitor electrically connected to a primary side of the transformer in series; an output capacitor electrically connected to a secondary side of the transformer in series; an output diode electrically connected to two output terminals of the transformer isolated gate driver in parallel; an active switch electrically connected to the output capacitor in series and having an input electrode electrically connected to a first terminal of the secondary side of the transformer, and a first electrode electrically connected to .a second terminal of the secondary side of the transformer; and the secondary side of the transformer, the output capacitor, the output diode, and the active switch are electrically connected in series, and when the PWM signal is disappeared, by discharging a plurality of charges across an equivalent input capacitor of the power switch through the secondary side of the transformer, the output capacitor, and a body diode of the active switch, a driving signal on the power switch is disappeared.
Preferably, the two polarities of the transformer are the same.
Preferably, the two polarities of the primary and secondary windings of the transformer are reversed.
Preferably, the power switch is a MOSFET.
Preferably, the active switch is a small power MOSFET, and the input electrode, the first electrode, and the second electrode of the active switch are a gate, a source, and a drain of a MOSFET respectively.
Preferably, the output capacitor is electrically connected between a cathode of the output diode and the input electrode of the active switch.
Preferably, the output capacitor is electrically connected between an anode of the output diode and the second electrode of the active switch.
Preferably, one selected from a group consisting of the input capacitor, the output capacitor, and the two output terminals of the transformer isolated gate driver is electrically connected to a resistor in parallel selectively.
Preferably, an input diode is electrically connected to the input capacitor in parallel, wherein an anode of the input diode is connected to a first end of the input capacitor connecting with the primary winding of the transformer, and a cathode of the input diode is connected to a second end of the input capacitor.
According to another aspect of the present invention, a transformer isolated circuit electrically connected to a PWM driver and a power switch for realizing the driving of the power switch includes: a transformer; an input capacitor electrically connected to a primary winding of the transformer in series; an output capacitor electrically connected to a secondary side of the transformer in series; an output diode electrically connected to two output terminals of the transformer isolated gate driver in parallel; an active switch electrically connected to the output capacitor in series and having an input electrode electrically connected to a first terminal of the secondary side of the transformer, and a first electrode electrically connected to a second terminal of the secondary side of the transformer; and the secondary side of the transformer, the output capacitor, the output diode, and the active switch are electrically connected in series.
Preferably, the two polarities of the primary and secondary windings of the transformer are the same.
Preferably, the two polarities of the primary and secondary windings of the transformer are reversed.
The present invention may best be understood through the following descriptions with reference to the accompanying drawings, in which: